1. Field of the Invention
The present invention relates to a supercharged direct-injection engine which comprises a supercharging device for compressing intake air, and an injector for directly injecting fuel into a combustion chamber.
2. Description of the Background Art
Heretofore, there has been known a spark-ignition gasoline engine designed to be operated with a predetermined lean fixed air-fuel ratio (e.g., about 25), in an engine operating region where an intake air amount is equal to or less than a given value, and operated with a combustible lean air-fuel ratio greater than the fixed air-fuel ratio, in an engine operating region where the intake air amount is greater than the given value, as disclosed, for example, in JP 2005-30255A.
Specifically, the gasoline engine disclosed in the above patent document is designed to compress intake air using a turbocharger, at least in an engine operating region where the engine is operated with the combustible lean air-fuel ratio, to increase the air-fuel ratio to become greater (leaner) than the fixed air-fuel ratio. This provides an advantage that a lean air-fuel ratio is achieved in a wide engine operating region to allow fuel consumption or NOx emission to be reduced.
While the above patent document makes no mention of how to ensure combustion performance in a wide engine operating region under lean air-fuel ratios, it is often that a technique of directly injecting fuel from an injector into a combustion chamber to perform stratified combustion is employed to achieve lean combustion (lean burn) in a spark-ignition gasoline engine. Specifically, the technique is intended to directly inject high-pressure fuel from an injector into a combustion chamber in such a manner that the injected fuel is unevenly distributed around a spark plug and formed as a layer having a combustible air-fuel ratio to ensure combustion performance, while achieving a lean air-fuel ratio for the entire combustion chamber.
However, in combustion of an air-fuel mixture under a condition that a fuel rich atmosphere is locally formed around a spark plug in the above manner (stratified combustion), a combustion temperature becomes higher to promote NOx generation, which causes a problem of being unable to obtain a sufficient NOx-reduction effect.